1. Field of the Invention
The present invention relates to a method of producing a semiconductor device, and more particularly to a method of producing a semiconductor device for forming a trench isolation structure.
2. Description of the Background Art
Trench isolation is known as one of the isolation structures in semiconductor devices. In order to form a bird""s beak in the trench isolation, an inner wall oxidation process for oxidizing an exposed inner wall of the trench is carried out after the trench is formed.
Hereafter, one example of such a method of producing a semiconductor device for forming a trench isolation structure according to a prior art will be described. First, referring to FIG. 18, a pad oxide film made of a silicon oxide film is formed on a silicon substrate 101. A polysilicon film is formed on the pad oxide film. A silicon nitride film is formed on the polysilicon film. A photoresist pattern (not illustrated) is formed on the silicon nitride film.
With the use of the photoresist pattern as a mask, an anisotropic etching treatment is carried out on the silicon nitride film, the polysilicon film, and the pad oxide film to expose a surface of the silicon substrate 101, thereby forming a mask pattern including a silicon nitride film 104a, a polysilicon film 103a, and a pad oxide film 102a for forming a trench. With the use of the mask pattern as a mask, an anisotropic etching treatment is carried out on the silicon substrate 101 to form a trench 106.
Subsequently, referring to FIG. 19, a thermal treatment (inner wall oxidation process) is carried out to form an oxide film 107 on the exposed surface of the trench 106, on the side surface of the pad oxide film, and on the side surface of the polysilicon film 103a. 
Next, referring to FIG. 20, a silicon oxide film 108 is formed to fill the trench 106. Thereafter, a chemical mechanical polishing treatment is carried out to polish the surface of the silicon oxide film 108. Next, referring to FIG. 21, a wet etching treatment is carried out on the silicon oxide film 108 to expose the silicon nitride film 104a almost completely.
Thereafter, the exposed silicon nitride film 104a is removed, and further the polysilicon film 103a and the pad oxide film 102a are successively removed. Thus, referring to FIG. 22, a bird""s beak is formed in the trench isolation oxide film 109.
Next, referring to FIG. 23, gate electrodes 113 are formed to cross over element forming regions 114 formed on the surface of the silicon substrate 101 by the trench isolation oxide film 109. Thereafter, a semiconductor device such as a dynamic random access memory (DRAM) is formed, for example, by forming memory cells or the like in the element forming regions 114.
However, the aforementioned conventional production method involves the following problem. As described above, the inner wall oxidation shown in FIG. 19 is carried out in a state in which the side surface of the polysilicon film 103a is generally flush (coplanar) with the side surfaces of the pad oxide film 102a and the trench 106, as shown in FIG. 18.
In the inner wall oxidation, the oxidation rate of the polysilicon film 103a is substantially at the same level as the oxidation rate of the silicon substrate 101 (the surface of the trench 106), while the oxidation rate of the pad oxide film 102a is smaller than the oxidation rate of the polysilicon film 103a. For this reason, an oxide film grows faster on the surface of the silicon substrate 101 and on the surface of the polysilicon film 103a than on the surface of the pad oxide film 102a, thereby forming an overhung configuration in which the part of the oxide film that has grown on the side surface of the polysilicon film 103a is hung over the oxide film that has grown on the side surface of the pad oxide film 102a. 
As a result of this, referring to FIG. 19, a recess 120 is formed between the part that has grown on the surface of the silicon substrate 101 and the part that has grown on the surface of the polysilicon film 103a in the oxide film 107. When a buried oxide film 108 is buried in the trench 106 in the step shown in FIG. 20 in a state in which the recess 120 has been formed, the part of the recess 120 is not filled with the buried oxide film 108 and remains as voids 121.
Referring to FIG. 22, these voids 121 form recesses 122 in the trench isolation oxide film 109 after the silicon nitride film 104a, the polysilicon film 103a, and the pad oxide film 102a are successively removed. If an attempt is made to form gate electrodes 113 in a state in which such recesses 122 have been formed in the trench isolation oxide film 109, an electrically conductive material 116 existing in the recesses 122, which material is for forming the gate electrodes 113, cannot be removed in patterning the gate electrodes 113.
For this reason, referring to FIG. 23, adjacent gate electrodes may possibly be short-circuited with each other by the electrically conductive material 116 that is remaining in the recesses 122. As a result of this, a desired operation may not be achieved in the semiconductor device.
The present invention has been made in order to solve the aforementioned problem of the prior art, and an object thereof is to provide a method of producing a semiconductor device that prevents voids from being formed in the trench isolation structure.
A method of producing a semiconductor device according to the present invention includes the following steps. A first film having a dielectric property, a second film having an oxidation property different from that of the first film, and a layer serving as a mask material are successively formed on a semiconductor substrate. A mask pattern for forming a trench in the semiconductor substrate is formed by performing an etching treatment on the first film, the second film, and the layer serving as a mask material. A trench is formed by performing an etching treatment on the semiconductor substrate using the mask pattern as a mask. An etching treatment is performed so that a position of a side surface of the second film exposed on a side surface of the mask pattern will be retreated from a position of a side surface of the first film. An oxide film is formed on a surface of the first film and the trench including the retreated side surface of the second film by performing a thermal treatment. A buried dielectric film is formed on the oxide film so as to fill the trench. In the retreating step of retreating the position of the side surface of the second film from the position of the side surface of the first film, the side surface of the second film is retreated in advance so that the oxide film that has grown on the side surface of the second film may not be hung over the oxide film that has grown on the side surface of the first film after the thermal treatment step of forming the oxide film by thermal treatment is performed.
According to this production method, the side surface of the second film is retreated in advance for a predetermined length so that the oxide film that has grown on the side surface of the second film may not be hung over the oxide film that has grown on the side surface of the first film. Therefore, voids can be prevented from being formed when the buried oxide film is buried in the trench. This can prevent generation of etching residues in a trench isolation structure, which may occur in a conventional semiconductor device, e.g. in forming two gate electrodes extending in parallel so as to cross the trench isolation structure, thereby preventing the two gate electrodes from being electrically short-circuited.
Preferably, the second film is retreated by dry etching in the retreating step.
This can facilitate control of the retreated amount of the second film (the amount of etching) and provides a better uniformity of the retreated amount in a surface of the semiconductor substrate.
Preferably, the second film is retreated by wet etching in the retreating step. This can shorten the processing time.
Further, the retreating step is preferably performed before the step of forming the trench.
This can prevent etching of the inner wall part of the trench by retreating the side surface of the second film before forming the trench, thereby forming an isolation oxide film including an oxide film and a buried oxide film having a desired shape.
Furthermore, it is preferable that the first film, the second film, and the layer serving as a mask material specifically include a silicon oxide film, a polysilicon film, and a silicon nitride film, respectively.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.